1. Field of Invention
This invention relates to the bonding of input/output (I/O) electrical connection pins, and other components, to a chip carrying substrate in an electronic system. More particularly, this invention relates to an electronic circuit interconnection bonding technique for fabrication of chip carrying substrates and more specifically to the bonding of pins thereto in a manner compatible with continuous reheating of chips.
In the semiconductor packaging technology it is frequently necessary and/or advantageous to affix input/output pins and other current carrying elements to the surface of a substrate having an interconnection network of current carrying lines. A specific example is bonding pins to I/O pads on the bottom surface of a multilayer ceramic (MLC) substrate. The pads are joined to an internal metallurgy system that is connected to integrated circuit devices mounted on the top surface of the substrate. Such a semiconductor package is described and claimed in U.S. Pat. No. 4,245,273. In the manufacture and operation of complex, multi-device packages, it is necessary to repair and/or rework the modules by removing the cover and replacing devices that are defective or defectively joined. This operation inevitably requires reheating of the module and more specifically the substrate. The I/O pins on the substrate should desirably be brazed to the substrate with an alloy that is not significantly affected by reheating of the substrate to temperatures necessary to re-melt the solder. It is conventional to use braze preform or paste containing a particular alloy to bond pins. The preforms and paste are normally formed of 80-20 AuSn. This alloy is stable, and has a melting point above the melting point of lead/tin solder. Thus, the bond between the pins and the substrate is not melted during rework. However, the cost of this brazing alloy is very expensive because the price of Au is high and the alloy contains a high percentage of Au.
Because of the melting point requirements, the necessity for forming a strong and stable bond for attaching pins that is not affected by environmental conditions, the number of brazing alloys and metals for such use is quite limited. Ag brazing alloys, which are less expensive than Au brazing alloys have not been used because of their high susceptibility for causing failure by electromigration.
A process for forming a strong stable bond between a pin, or other current carrying elements, and a substrate that is also inexpensive is a present pressing need in the semiconductor technology industry.